Most power plants utilize a water/steam boiler system to energize a generator turbine and produce electricity. Because of the extremely high pressures and temperatures involved in the process, dissolved gasses and solids in the system water dramatically increase the deleterious effect on boilers steel and other metallic components.
A boiler circuit is largely but not a completely closed system, specifically, because solids are introduced into the water through make-up water, erosion, corrosion and other means. They need to be removed periodically or continuously to prevent build up from negatively impacting the function and efficiency of the system. Main boiler blow down is used to flush out the solids but also releases a small percentage of water from the system that needs to be replaced. Further, a small amount of water is lost in the form of steam vented out of a deaerator and through soot blowers in coal fired plants.
Make-up water subsystems are employed to replace the flushed water. FIG. 1 illustrates a typical power plant steam boiler system and its prior art make-up water subsystem. The make-up water subsystem acts to remove most of the dissolved solids in the make-up water to a level below 250 parts per million depending on boiler pressure prior to introduction into the main boiler 110. Prior art systems typically employ various chemical compounds using both cation and anion ion exchange resins and/or reverse osmosis (RO) membranes to remove dissolved solids from unprocessed well (or raw) water. The process results in deionized or RO water that is largely solids-free.
Referring to FIG. 1, the operation of a typical power plant steam boiler system 100 is described with reference to the flow of steam and water in the boiler loop. The water contained in the main boiler 110 is heated to extremely high temperatures (up to 1050 degrees F.) and maintained at extremely high pressures (from 1200-3550 psig). A suitable heat source 111 is provided to heat the boiler and steam contained therein and may be fueled by coal, natural gas, nuclear or any other suitable energy source. The superheated pressurized steam is funneled to an electric turbine generator 112 through one or more connecting pipes 118. The thermal energy of the steam is converted to mechanical energy spinning a turbine which in turn is converted to electricity. The spent steam is directed through a conduit exiting the generator and into a condenser tank 114 wherein the steam condenses into water at about 90-120 degrees F. Typically, the tank is maintained at low pressure or even a vacuum relative to ambient.
The condensed water is then fed into a deaerator 116 by one or more conduits 122. A small amount of the steam from the pressurized steam pipes 118 extending between the main boiler and the electric generator is diverted and piped into the deaerator. The deaerator includes a pressure release vent and through this maintains the pressure in it to a desired level of about 5-60 psi depending on the particular application. As the cool condensed water is received in the deaerator, the steam heats it to 250-300 F and deaerates it freeing and venting any previously dissolved oxygen and other gasses to atmosphere through the vent. The deaerated water pools at the bottom of the deaerator and is fed back into the boiler to repeat the cycle.
As mentioned above a small amount of water is lost from the otherwise closed boiler loop when steam is vented out of the deaerator's vent. A much larger amount of water is removed from the system through boiler blown down. Overtime, small amounts of solids, such as from the walls of the boiler and pipes, dissolve into the boiler water and steam. If the level of dissolved solids reaches too high a level the contaminants can accelerate the corrosion and erosion of internal surfaces within the closed system. Accordingly, a blown down valve 126 and associated piping 128 is provided near the base of the main boiler where any solids may collect. Periodically, and when sensors located in the system indicated that the solid level is close to or exceeds 250 ppm, the blow down valve is opened to release a portion of the water along with particulate and dissolved solid contained in it to atmosphere. In addition to a loss of system water, which must be made up and replaced, the heat energy contained in the water is lost. While the lost heat energy is low relative to the energy output of the system, the value of the energy over the course of months and years can represent a measurable and significant amount. Furthermore, the treatment of make-up water to remove dissolved solids and other contaminants therefrom represents a sizable and significant cost over time.
A typical prior art make-up water subsystem 130 utilizes water from a well 132 or other suitable source. As can be appreciated well water wherein chlorine and other additives have not been introduced into the water is preferred since any additives have to be removed by the make-up water subsystem before the water can be introduced into the boiler loop. The water make-up subsystem includes a means for pumping water from a well 132, a means for scrubbing the water to remove particulate and dissolved solids, and a pipe 134 or pipes coupled with the boiler loop to introduce the make-up water therein. In the illustrated prior art subsystem, the means for scrubbing the water comprises a series of tanks 136 & 138 filled with cation resins and anion resins through which the water passes and solids are removed. In other makeup water subsystems reverse osmosis membranes can be used in combination with or in place of the cation and anion tanks.
While the prior art systems are very effective, over time they can be expensive requiring tens of thousands of dollars in chemicals, lost water, or reverse osmosis filters and membranes annually for a small to medium-sized power plant. Additional inefficiencies occur from the loss of heat energy in the flushing of very high temperature water during boiler blow down.